Ligand control of magnetic ordering temperature in copper-pyrazine square lattice antiferromagnets*

Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional (Q2D) antiferromagnets: [Cu(HF$_2$)(pyz)$_2$]ClO$_4$ [pyz = pyrazine], [Cu$L_2$(pyz)$_2$](ClO$_4$)$_2$ [$L$ = pyO = pyridine-N-oxide and 4-phpyO = 4-phenylpyridine-N-oxide). These possess equivalent 2D [Cu(pyz)$_2$]$^{2+}$ nearly square layers, but show interlayer spacings from 6.57~\AA ~to 16.78~\AA, dictated by the axial ligands. Structural and magnetic properties are derived from x-ray diffraction, ESR, pulsed-field magnetometry and muon-spin rotation, and compared to those of the prototypical 2D magnetic polymer Cu(ClO$_4$)$_2$(pyz)$_2$. We find that the 2D exchange coupling remains largely unaffected by the axial ligand substitution, while the magnetic ordering temperature decreases slowly with increasing layer separation. Experimental data are compared to theory, including DFT.